INTERSTELLAR GRAINS IN METEORITES .1. ISOLATION OF SIC, GRAPHITE, AND DIAMOND - SIZE DISTRIBUTIONS OF SIC AND GRAPHITE

A procedure has been developed for isolating three types of interstellar grains from primitive meteorites, in >90% purity and yields of generally greater-than-or-equal-to 70%, and is here applied to the Murchison C2M meteorite. Silicates are dissolved in HF-HCl kerogen (macromolecular organic matter) is destroyed by Cr2O7=, KOH, and H2O2, and microdiamonds (approximately 400 ppm) are recovered as a colloid. Graphite (<1 ppm) is isolated by density and size separations. Spinel in the residue is dissolved in H2SO4, leaving SiC (approximately 6 ppm), hibonite, and corundum. The size distribution of SiC has been measured in the range 0.2 to 6 mum. Over part of this range, it can be fit either to a power-law or a log-normal distribution, but the deficiency of small grains strongly favors the latter. Statistics are more limited for graphite spherules, but they, too, follow a log-normal distribution, and so does interstellar diamond (LEWis et al., 1989). Apparently the primary condensation process in stellar atmospheres consistently yields a log-normal distribution. The power-law distribution commonly inferred for interstellar grains may have been produced by secondary processes such as fragmentation.